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Re: [OM] [OT] infrared flash bulbs?

Subject: Re: [OM] [OT] infrared flash bulbs?
From: "Jeff Keller" <jrk_om@xxxxxxxxxxx>
Date: Wed, 2 Jul 2003 21:06:28 -0700
Perhaps the appropriate response is way cool!

With finger temperatures going through phone books, there's not much
point in looking for corresponding film. Quite interesting, thanks

-jeff

----- Original Message -----
From: "Kennedy McEwen" <rkm@xxxxxxxxxxxxxxxxxxxx>

> It wouldn't look much different from how it does in the visible band.
> The "infrared" band usually used in photography, and certainly used by
> the image convertor tube on a Nokta or similar is limited to below 1um
> in wavelength.  Looking at Planck's Law, black body radiation peaks at
> 1um for temperatures of about 3200degC, and your Christmas tree lights
> cool down from that temperature within a fraction of a second when
they
> are turned off.
>
> Lots of folk use this near infrared band for "covert" surveillance and
> animal photography at night is one such application - but you do need
a
> source of illumination, such as a spotlight or flash, and that means
it
> isn't covert for anyone who has a similar camera.  If you look at most
> building security video cameras (especially the external types) they
> will have a deep red spotlight next to them.  That is because the
> response of a silicon CCD expends into this band - silicon will go up
to
> 900nm easily and can be extended to almost 1.2um if suitably treated
at
> manufacture.  If you look at such spotlights at night there is a faint
> visible red glow, due to a small amount of light in the visible band
> being emitted.
>
> Now, if you had the technology to go out to 5um or even 14um then its
a
> different story.  No illumination source needed because peak emissions
> occur for objects around normal terrestrial temperatures.  This is
real
> covert surveillance - with obvious uses.
>
> I work with stuff operating at these wavelengths every day, in fact i
> have been designing a lot of it for a few decades, and you might see
the
> lights on the Christmas tree several HOURS after they are turned off.
In
> fact, you would be able to see the heat from your finger on the light
> switch for several minutes after they had turned off.  And see your
hot
> footprints on the floor where you walked over to the switch and where
> you walked afterwards!
>
> A fun trick I show folk that have never seen such cameras is the
> telephone directory demo.  Put your open hand on the front cover of a
> telephone directory for about 10 seconds and then open it about 50-100
> pages in - there is the image from the heat of your hand, with fingers
> clearly visible.  Wait a few seconds and then go another 50 pages -
the
> image is there as well.  If you time it just right you can catch the
> thermal wave as it propagates right through from the front cover to
the
> back although it gets less distinct after a thousand pages or more.
>
> A lot of my colleagues refer to the sub-1um band as "infra-pink" - its
> just barely infrared at all.  ;-)
> --
> Kennedy

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